After supporting the Rock Band platform with new DLC tracks every week for 275 consecutive weeks, Harmonix has announced it is no longer going to focus on releasing DLC for Rock Band past April 2nd, 2013. The resources previously used for putting out the DLC will be diverted to new games Harmonix has in the works.
This doesn't really affect the current DLC or anything - the 4000+ songs already released will all still be there. There just won't be any new stuff. It also isn't a 100% end to DLC - they may release more down the road depending on if they get the right band or right offer - but they currently have no more DLC scheduled for release past April 2nd. The Rock Band Network, where users can create and submit their own tracks, will continue to function (on Xbox 360 at least, the PS3 version doesn't work the same way) as normal, so there will still be new content through RBN.